1. Field of the Invention
The present invention relates to a sheet detecting device, in particular to a sheet detecting device for detecting presence or absence of a printing sheet, a right side or wrong side of a printing sheet set in an image forming device or the like and for detecting the kind of the printing sheet to be fed in the sheet detecting device along a sheet carrying path for use in the image forming device or the like.
2. Description of the Prior Art
In an image forming device of a photosensitive and pressure sensitive type which has been developed in recent years, there has been used a copying sheet with resin material or the like coated thereon for printing a copying image. For example, there is used a photosensitive sheet of which the right side surface is coated with microcapsules containing light hardener materials and image forming materials. At the same time of this, there is also used an image receiving sheet of which the right side surface is coated with color developer for coloring the image receiving sheet by reaction with the image forming material and is coated with thermoplastic resin together with the photosensitive sheet. In the case of using such a pair of image receiving sheet and photosensitive sheet, the pressure developing process is carried out by piling up the image forming sheet with the photosensitive sheet in such a manner that the coated surface of the imaging sheet is faced to the coated surface of the photosensitive sheet. Therefore, it is an important matter that the image receiving sheet is carried along the sheet carrying path with its right side and wrong side surfaces correctly set.
Therefore, generally in such an image forming device as described above, a pair of sensors for example, are provided in front of a timing roller for feeding an image receiving sheet to a pressure developing process, thereby judging whether or not the right side and wrong side are correctly set so as to judge the kind of the sheet. As a method of judgment of the right side and wrong side of the image receiving sheet, for example, there are provided a pair of reflection type photosensors in both of the upper and lower sides of the sheet carrying path so as to detect the reflectances of the light reflected by the right side and wrong side of the sheet, thereby judging the right side and wrong side of the sheet by detecting the difference between the reflectances of the right side and wrong side of the sheet.
Moreover, in such an image forming device of a photosensitive and pressure sensitive type as described above, if the pressure developing process is carried out under the condition that there is not fed an image receiving sheet, the microcapsules coated on the surface of the photosensitive sheet are broken so that the image forming materials contained in the microcapsules flow out so as to stain a press roller. Consequently, the quality of the image is deteriorated in the subsequent image forming operation due to a stain on the image receiving sheet with the image forming materials. That is the reason why it is necessary to detect presence or absence of an image receiving sheet, and in general, by providing another sensor in front of the timing roller on the sheet carrying path other than a pair of sensors as mentioned above, it has been judged whether an image receiving sheet is present or absent.
Consequently, a sensor is required for judging presence or absence of an image receiving sheet other than the sensors for judging the right side and wrong side and the kind of the image receiving sheet in a narrow space defined between the sheet feeding roller and the timing roller, so that the arrangement of the sensors is complicated and there has been a problem that the cost of the device is raised due to the increment of the space for accommodating the sensors and the increment of the parts thereof.